Melanic variation underlies aposematic color variation in two hymenopteran mimicry systems

<div><p>The stinging hymenopteran velvet ants (Mutillidae) and bumble bees (Apidae: <i>Bombus</i> spp.) have both undergone extensive diversification in aposematic color patterns, including yellow-red hues and contrasting dark-light body coloration, as a result of Müllerian mimicry. Understanding the genetic and developmental mechanisms underlying shifts in these mimetic colors requires characterization of their pigmentation. In this study, a combination of solubility, spectrophotometry, and melanin degradation analysis are applied to several color forms and species of these lineages to determine that orange-red colors in both lineages are comprised of primarily dopamine-derived pheomelanins. Until a few recent studies, pheomelanins were thought not to occur in insects. These results support their potential to occur across insects and particularly among the Hymenoptera. Shifts between black and orange-red colors, such as between mimetic color forms of bumble bee <i>Bombus melanopygus</i>, are inferred to involve modification of the ratios of dark eumelanins to red pheomelanins, thus implicating the melanin pathway in mimetic diversification. This discovery highlights the need to focus on how pheomelanins are synthesized in the insect melanin pathway and the potential for new pigments to be found even in some of our most well-known insect systems.</p></div

Results of solubility tests, spectrophotometry, and TLC on sampled velvet ants and bumble bees.

<p>Results of solubility tests, spectrophotometry, and TLC on sampled velvet ants and bumble bees.</p

Reflectance (A) and absorbance (B) of specimens from Fig 1.

<p>Colors represent the color of the setal samples tested, measured from RGB data from photographs. Dashed lines in 2B indicate absorbance from melanin precipitate of mutillid B1. To enable better comparison of relative differences in shape of the absorbance and reflectance curves, reflectance curves have been standardized to a common value at 750 nm and absorbance curves have been adjusted to yield comparable levels between 225 and 275 nm, except in a few cases with especially low absorbances (D3,D4).</p

Specimens studied.

<p>B1—P3 were used for reflectance and absorbance measurements and represent: <b>B1</b> = <i>Dasymutilla bioculata</i>, dark orange/red form [USA: Utah: Cache Co., Hyrum; 41.63, 111.86; 7/8/05;T. Bond]; <b><i>B4</i></b> = <i>Dasymutilla bioculata</i>, dark orange/red form [USA: Utah: Cache Co., Hyrum; 41.63, 111.86; 8/11/05; T. Bond]; <b><i>B5</i></b> = <i>Dasymutilla bioculata</i>, medium orange form [USA:Utah: Emery Co., 25km NNE Hanksville, Gilson Butte Well; 38.58, -110.58; 24.VII.2001; M. Hauser]; <b><i>B6</i></b> = <i>Dasymutilla bioculata</i>, light orange form [USA: Utah; Garfield Co.; 37.34, -111.06; 25-July-2001; RW Baumann, RD Gordon, IS Winkler]; <b><i>S2</i></b> = <i>Dasymutilla scitula</i> [USA: Utah; Garfield Co., Calf Creek; 37.83, -111.42; VII-30-1982; Griswold/Parkers]; <b><i>G3</i></b> = <i>Dasymutilla gloriosa</i> [USA: Utah; Washington Co., Werner Valley, 8mi SE St. George, 37.02533, -113.43408; 25-30.viii.2010; J. Wilson]; <b><i>D3</i></b> = <i>Dasymutilla satanas</i>, orange form [USA: Nevada; Rock Valley, Nye Co.; 36.63, -116.31; 7/28/1965; EF Dailey-Attrop]; <b><i>D4</i></b> = <i>Dasymutilla satanas</i>, light orange form [USA: Arizona, Parker Dam; 34.30, -114.13; 7-IX-1963]; <b><i>P3</i></b> = <i>Psorthaspis portiae</i> [USA: Arizona; Cochise Co., Rte. 666 W. of Wilcox; 32.15, -109.93; 17-July-1991; BP Harris]. Remaining specimens are representatives of species used for degradation analysis as well as absorbance and reflectance measurements. <b>DO</b> = <i>Dasymutilla occidentalis</i>. All specimens used were female. Specimens were cropped from their original image. DO and the bumble bees were photographed separately from other mutillids and color was standardized between images. Their scaling relative to other specimens is approximate. Specimens in the first row + G3 were increased to 125% of the scale of other specimens to improve visualization.</p

Criteria used for assessing pigment types extracted from insect cuticle based on solubility, UV fluorescence, and spectral shape.

<p>While spectral wavelengths shift, general spectral shape is similar across the pigment class. Some deviation from this model could occur given side groups of some members of each class.</p

Tyrosinase-Catalyzed Oxidation of the Leukoderma-Inducing Agent Raspberry Ketone Produces (<i>E</i>)‑4-(3-Oxo-1-butenyl)-1,2-benzoquinone: Implications for Melanocyte Toxicity

The exposure of human skin to 4-(4-hydroxyphenyl)-2-butanone (raspberry ketone, RK) is known to cause chemical/occupational leukoderma. RK has a structure closely related to 4-(4-hydroxyphenyl)-2-butanol (rhododendrol), a skin whitening agent that was found to cause leukoderma in the skin of consumers in 2013. Rhododendrol is a good substrate for tyrosinase and causes a tyrosinase-dependent cytotoxicity to melanocytes, cells that are responsible for skin pigmentation. Therefore, it is expected that RK exerts its cytotoxicity to melanocytes through the tyrosinase-catalyzed oxidation to cytotoxic <i>o</i>-quinones. The results of this study demonstrate that the oxidation of RK by mushroom tyrosinase rapidly produces 4-(3-oxobutyl)-1,2-benzoquinone (RK-quinone), which is converted within 10–20 min to (<i>E</i>)-4-(3-oxo-1-butenyl)-1,2-benzoquinone (DBL-quinone). These quinones were identified as their corresponding catechols after reduction by ascorbic acid. RK-quinone and DBL-quinone quantitatively bind to the small thiol <i>N</i>-acetyl-l-cysteine to form thiol adducts and can also bind to the thiol protein bovine serum albumin through its cysteinyl residue. DBL-quinone is more reactive than RK-quinone, as judged by their half-lives (6.2 min vs 10.5 min, respectively), and decays rapidly to form an oligomeric pigment (RK-oligomer). The RK-oligomer can oxidize GSH to GSSG with a concomitant production of hydrogen peroxide, indicating its pro-oxidant activity, similar to that of the RD-oligomer. These results suggest that RK is cytotoxic to melanocytes through the binding of RK-derived quinones to thiol proteins and the pro-oxidant activity of the RK-oligomer

Evolution-BSO_sparrows-Dryad

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Contents of eumelanin and pheomelanin in hairs of spiny mice (<i>Acomys cahirinus</i>) from “Evolution Canyon” I, Israel.

<p>(A) in the opposite slopes, (B) across stations. Means with the same letter are not significantly different according to Waller-Duncan k-ratio t-test.</p

Habitat divergence in “Evolution Canyon”.

<p>(A) photo showing part of the “African” slope (AS)/south-facing slope (SFS) of EC, characterized by light terra rossa soil with a stretch of grass cover that generates a yellow-brownish background. (B) Photo of part of the “European” slope (ES)/north-facing slope (NFS), characterized by dark terra rossa soil and shady, humus-laden dark background.</p

“Evolution Canyon” (EC), Nahal Oren, Israel.

<p>(A) aerial view and (B) cross-section showing the opposing slopes. Note the two habitat types; dry, open habitat (with light colored background) on the “African” slope and humid, closed habitat (with dark-colored background) in the “European” slope.</p